1. Field of the Invention
The present invention relates to a servo control apparatus suitable for use in a shaft friction compensation technique for a positioning control that is performed in, for example, a situation that high friction torque acts on a drive shaft for rotating a load.
2. Background Art
In systems for tracking or monitoring a subject, the direction of a camera is caused to follow the subject by rotating a base to which the camera is fixed by motor driving. Where a follow-up system is used outdoors, to protect electronic devices etc., a sealing member for a dust-proof or drip-proof purpose is attached to the motor shaft. While the motor shaft is being rotated, the sealing member causes friction to act on the motor shaft. Therefore, positioning is performed about the rotation angle of the motor shaft, the positioning accuracy is lower than in a case that no sealing member is attached to the motor shaft.
If the follow-up system can measure the value of friction torque acting on the motor shaft, usually, rotation angle positioning is performed by using a control system which feeds back the error of the rotation angular position of the motor shaft from a target position. The motor shaft is squeezed with the sealing member to such an extent that its rotation is not hindered. And the motor shaft is not rotated and a positioning error occurs unless the controlled variable is fed back so that drive torque that is higher than measured friction torque is output. Since the motor does not start to rotate until drive torque that is higher than the friction torque is generated after issuance of an instruction to start a positioning control. This results in increase of the positioning settlement time. Therefore, in conventional follow-up systems, a control system is designed so that the friction effect is exercised by increasing the gain of a positioning control system taking a torque loss due to the friction by the sealing member into consideration.
Among conventional techniques is a servo technique which suppresses vibration of a load of a vibration-system mechanism using an observer for estimating disturbance acting on a motor (refer to JP-A-2004-199495, for example). Techniques for increasing the positioning accuracy have also been proposed in the following documents: (a) JP-A-2004-199495, (b) Koji Umeno: “Estimation of Vehicle State Quantity Based on Disturbance Observer,” R & D Review of Toyota Central Research and Development Laboratories, Inc., Vol. 29, No. 4, December 1994, and (c) Yuji Ide et al.: “Control Technologies Effective in Servo Systems,” Sanyo Denki Technical Report, No. 14, November 2002.
However, in design methods using the conventional techniques, it is difficult to effectively suppress shaft friction on a motor shaft because the rigidity of the motor shaft that is attached to a load has a limit and hence a shaft resonance characteristic prevents the gain of a control system from being increased sufficiently. On the other hand, it is difficult for design methods with high rigidity of the motor shaft to reduce the size and weight of a follow-up system.
Designs are possible that utilize a method of detecting a quantity reflecting influence of friction by introducing an additional sensor such as a torque sensor and adding, to a positioning control system, a feedback control loop for feeding back this quantity. However, the introduction of the additional sensor not only increases the cost of the follow-up system but also makes it necessary to adjust the control system for sensor-dependent variation. The system is thus rendered complex.
In addition, the state of the sealing surface varies as the sealing member slidingly contacts the motor shaft repeatedly and such variation with age may increase the friction. As such, follow-up systems using a sealing member may be deteriorated rapidly in performance.
The present invention therefore provides a servo control apparatus capable of compensation control for suppressing shaft friction on a motor shaft in a situation that the motor shaft has a certain resonance characteristic.